diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 23f594267..5b1fb3668 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -205,6 +205,7 @@ set(SLIC3R_TEST_SOURCES
     ${TESTDIR}/libslic3r/test_trianglemesh.cpp
     ${TESTDIR}/libslic3r/test_config.cpp
     ${TESTDIR}/libslic3r/test_support_material.cpp
+    ${TESTDIR}/libslic3r/test_fill.cpp
     ${TESTDIR}/libslic3r/test_flow.cpp
     ${TESTDIR}/libslic3r/test_model.cpp
     ${TESTDIR}/libslic3r/test_printgcode.cpp
diff --git a/src/test/libslic3r/test_fill.cpp b/src/test/libslic3r/test_fill.cpp
new file mode 100644
index 000000000..604da6dae
--- /dev/null
+++ b/src/test/libslic3r/test_fill.cpp
@@ -0,0 +1,433 @@
+#include <catch.hpp>
+#include "test_data.hpp"
+#include "Fill.hpp"
+/* 
+BEGIN {
+    use FindBin;
+    use lib "$FindBin::Bin/../lib";
+    use local::lib "$FindBin::Bin/../local-lib";
+}
+
+use List::Util qw(first sum max);
+use Slic3r;
+use Slic3r::Geometry qw(PI X Y scaled_epsilon scale unscale convex_hull);
+use Slic3r::Geometry::Clipper qw(union diff diff_ex offset offset2_ex diff_pl union_ex);
+use Slic3r::Surface qw(:types);
+use Slic3r::Test;
+
+sub scale_points (@) { map [scale $_->[X], scale $_->[Y]], @_ }
+
+{
+    my $print = Slic3r::Print->new;
+    my $surface_width = 250;
+    my $distance = Slic3r::Flow::solid_spacing($surface_width, 47);
+    is $distance, 50, 'adjusted solid distance';
+    is $surface_width % $distance, 0, 'adjusted solid distance';
+}
+
+{
+    my $filler = Slic3r::Filler->new_from_type('rectilinear');
+    $filler->set_angle(-(PI)/2);
+    $filler->set_min_spacing(5);
+    $filler->set_dont_adjust(1);
+    $filler->set_endpoints_overlap(0);
+    
+    my $test = sub {
+        my ($expolygon) = @_;
+        my $surface = Slic3r::Surface->new(
+            surface_type    => S_TYPE_TOP,
+            expolygon       => $expolygon,
+        );
+        return $filler->fill_surface($surface);
+    };
+    
+    # square
+    $filler->set_density($filler->min_spacing / 50);
+    for my $i (0..3) {
+        # check that it works regardless of the points order
+        my @points = ([0,0], [100,0], [100,100], [0,100]);
+        @points = (@points[$i..$#points], @points[0..($i-1)]);
+        my $paths = $test->(my $e = Slic3r::ExPolygon->new([ scale_points @points ]));
+        
+        is(scalar @$paths, 1, 'one continuous path') or done_testing, exit;
+        ok abs($paths->[0]->length - scale(3*100 + 2*50)) - scaled_epsilon, 'path has expected length';
+    }
+    
+    # diamond with endpoints on grid
+    {
+        my $paths = $test->(my $e = Slic3r::ExPolygon->new([ scale_points [0,0], [100,0], [150,50], [100,100], [0,100], [-50,50] ]));
+        is(scalar @$paths, 1, 'one continuous path') or done_testing, exit;
+    }
+    
+    # square with hole
+    for my $angle (-(PI/2), -(PI/4), -(PI), PI/2, PI) {
+        for my $spacing (25, 5, 7.5, 8.5) {
+            $filler->set_density($filler->min_spacing / $spacing);
+            $filler->set_angle($angle);
+            my $paths = $test->(my $e = Slic3r::ExPolygon->new(
+                [ scale_points [0,0], [100,0], [100,100], [0,100] ],
+                [ scale_points reverse [25,25], [75,25], [75,75], [25,75] ],
+            ));
+            
+            if (0) {
+                require "Slic3r/SVG.pm";
+                Slic3r::SVG::output(
+                    "fill.svg",
+                    no_arrows   => 1,
+                    expolygons  => [$e],
+                    polylines   => $paths,
+                );
+            }
+            
+            ok(@$paths >= 2 && @$paths <= 3, '2 or 3 continuous paths') or done_testing, exit;
+            ok(!@{diff_pl($paths->arrayref, offset(\@$e, +scaled_epsilon*10))},
+                'paths don\'t cross hole') or done_testing, exit;
+        }
+    }
+    
+    # rotated square
+    $filler->set_angle(PI/4);
+    $filler->set_dont_adjust(0);
+    $filler->set_min_spacing(0.654498);
+    $filler->set_endpoints_overlap(unscale(359974));
+    $filler->set_density(1);
+    $filler->set_layer_id(66);
+    $filler->set_z(20.15);
+    {
+        my $e = Slic3r::ExPolygon->new(
+            Slic3r::Polygon->new([25771516,14142125],[14142138,25771515],[2512749,14142131],[14142125,2512749]),
+        );
+        my $paths = $test->($e);
+        is(scalar @$paths, 1, 'one continuous path') or done_testing, exit;
+        ok abs($paths->[0]->length - scale(3*100 + 2*50)) - scaled_epsilon, 'path has expected length';
+    }
+}
+
+{
+    my $expolygon = Slic3r::ExPolygon->new([ scale_points [0,0], [50,0], [50,50], [0,50] ]);
+    my $filler = Slic3r::Filler->new_from_type('rectilinear');
+    $filler->set_bounding_box($expolygon->bounding_box);
+    $filler->set_angle(0);
+    my $surface = Slic3r::Surface->new(
+        surface_type    => S_TYPE_TOP,
+        expolygon       => $expolygon,
+    );
+    my $flow = Slic3r::Flow->new(
+        width           => 0.69,
+        height          => 0.4,
+        nozzle_diameter => 0.50,
+    );
+    $filler->set_min_spacing($flow->spacing);
+    $filler->set_density(1);
+    foreach my $angle (0, 45) {
+        $surface->expolygon->rotate(Slic3r::Geometry::deg2rad($angle), [0,0]);
+        my $paths = $filler->fill_surface($surface, layer_height => 0.4, density => 0.4);
+        is scalar @$paths, 1, 'one continuous path';
+    }
+}
+
+{
+    my $test = sub {
+        my ($expolygon, $flow_spacing, $angle, $density) = @_;
+        
+        my $filler = Slic3r::Filler->new_from_type('rectilinear');
+        $filler->set_bounding_box($expolygon->bounding_box);
+        $filler->set_angle($angle // 0);
+        $filler->set_dont_adjust(0);
+        my $surface = Slic3r::Surface->new(
+            surface_type    => S_TYPE_BOTTOM,
+            expolygon       => $expolygon,
+        );
+        my $flow = Slic3r::Flow->new(
+            width           => $flow_spacing,
+            height          => 0.4,
+            nozzle_diameter => $flow_spacing,
+        );
+        $filler->set_min_spacing($flow->spacing);
+        my $paths = $filler->fill_surface(
+            $surface,
+            layer_height    => $flow->height,
+            density         => $density // 1,
+        );
+        
+        # check whether any part was left uncovered
+        my @grown_paths = map @{Slic3r::Polyline->new(@$_)->grow(scale $filler->spacing/2)}, @$paths;
+        my $uncovered = diff_ex([ @$expolygon ], [ @grown_paths ], 1);
+        
+        # ignore very small dots
+        @$uncovered = grep $_->area > (scale $flow_spacing)**2, @$uncovered;
+        
+        is scalar(@$uncovered), 0, 'solid surface is fully filled';
+        
+        if (0 && @$uncovered) {
+            require "Slic3r/SVG.pm";
+            Slic3r::SVG::output(
+                "uncovered.svg",
+                expolygons      => [$expolygon],
+                red_expolygons  => $uncovered,
+                polylines       => $paths,
+            );
+            exit;
+        }
+    };
+    
+    my $expolygon = Slic3r::ExPolygon->new([
+        [6883102, 9598327.01296997],
+        [6883102, 20327272.01297],
+        [3116896, 20327272.01297],
+        [3116896, 9598327.01296997],
+    ]);
+    $test->($expolygon, 0.55);
+    
+    for (1..20) {
+        $expolygon->scale(1.05);
+        $test->($expolygon, 0.55);
+    }
+    
+    $expolygon = Slic3r::ExPolygon->new(
+        [[59515297,5422499],[59531249,5578697],[59695801,6123186],[59965713,6630228],[60328214,7070685],[60773285,7434379],[61274561,7702115],[61819378,7866770],[62390306,7924789],[62958700,7866744],[63503012,7702244],[64007365,7434357],[64449960,7070398],[64809327,6634999],[65082143,6123325],[65245005,5584454],[65266967,5422499],[66267307,5422499],[66269190,8310081],[66275379,17810072],[66277259,20697500],[65267237,20697500],[65245004,20533538],[65082082,19994444],[64811462,19488579],[64450624,19048208],[64012101,18686514],[63503122,18415781],[62959151,18251378],[62453416,18198442],[62390147,18197355],[62200087,18200576],[61813519,18252990],[61274433,18415918],[60768598,18686517],[60327567,19047892],[59963609,19493297],[59695865,19994587],[59531222,20539379],[59515153,20697500],[58502480,20697500],[58502480,5422499]]
+    );
+    $test->($expolygon, 0.524341649025257, PI/2);
+    
+    $expolygon = Slic3r::ExPolygon->new([ scale_points [0,0], [98,0], [98,10], [0,10] ]);
+    $test->($expolygon, 0.5, 45, 0.99);  # non-solid infill
+}
+
+{
+    my $collection = Slic3r::Polyline::Collection->new(
+        Slic3r::Polyline->new([0,15], [0,18], [0,20]),
+        Slic3r::Polyline->new([0,10], [0,8], [0,5]),
+    );
+    is_deeply
+        [ map $_->[Y], map @$_, @{$collection->chained_path_from(Slic3r::Point->new(0,30), 0)} ],
+        [20, 18, 15, 10, 8, 5],
+        'chained path';
+}
+
+{
+    my $collection = Slic3r::Polyline::Collection->new(
+        Slic3r::Polyline->new([4,0], [10,0], [15,0]),
+        Slic3r::Polyline->new([10,5], [15,5], [20,5]),
+    );
+    is_deeply
+        [ map $_->[X], map @$_, @{$collection->chained_path_from(Slic3r::Point->new(30,0), 0)} ],
+        [reverse 4, 10, 15, 10, 15, 20],
+        'chained path';
+}
+
+{
+    my $collection = Slic3r::ExtrusionPath::Collection->new(
+        map Slic3r::ExtrusionPath->new(polyline => $_, role => 0, mm3_per_mm => 1),
+            Slic3r::Polyline->new([0,15], [0,18], [0,20]),
+            Slic3r::Polyline->new([0,10], [0,8], [0,5]),
+    );
+    is_deeply
+        [ map $_->[Y], map @{$_->polyline}, @{$collection->chained_path_from(Slic3r::Point->new(0,30), 0)} ],
+        [20, 18, 15, 10, 8, 5],
+        'chained path';
+}
+
+{
+    my $collection = Slic3r::ExtrusionPath::Collection->new(
+        map Slic3r::ExtrusionPath->new(polyline => $_, role => 0, mm3_per_mm => 1),
+            Slic3r::Polyline->new([15,0], [10,0], [4,0]),
+            Slic3r::Polyline->new([10,5], [15,5], [20,5]),
+    );
+    is_deeply
+        [ map $_->[X], map @{$_->polyline}, @{$collection->chained_path_from(Slic3r::Point->new(30,0), 0)} ],
+        [reverse 4, 10, 15, 10, 15, 20],
+        'chained path';
+}
+
+for my $pattern (qw(rectilinear honeycomb hilbertcurve concentric)) {
+    my $config = Slic3r::Config->new_from_defaults;
+    $config->set('fill_pattern', $pattern);
+    $config->set('external_fill_pattern', $pattern);
+    $config->set('perimeters', 1);
+    $config->set('skirts', 0);
+    $config->set('fill_density', 20);
+    $config->set('layer_height', 0.05);
+    $config->set('perimeter_extruder', 1);
+    $config->set('infill_extruder', 2);
+    my $print = Slic3r::Test::init_print('20mm_cube', config => $config, scale => 2);
+    ok my $gcode = Slic3r::Test::gcode($print), "successful $pattern infill generation";
+    my $tool = undef;
+    my @perimeter_points = my @infill_points = ();
+    Slic3r::GCode::Reader->new->parse($gcode, sub {
+        my ($self, $cmd, $args, $info) = @_;
+        
+        if ($cmd =~ /^T(\d+)/) {
+            $tool = $1;
+        } elsif ($cmd eq 'G1' && $info->{extruding} && $info->{dist_XY} > 0) {
+            if ($tool == $config->perimeter_extruder-1) {
+                push @perimeter_points, Slic3r::Point->new_scale($args->{X}, $args->{Y});
+            } elsif ($tool == $config->infill_extruder-1) {
+                push @infill_points, Slic3r::Point->new_scale($args->{X}, $args->{Y});
+            }
+        }
+    });
+    my $convex_hull = convex_hull(\@perimeter_points);
+    ok !(defined first { !$convex_hull->contains_point($_) } @infill_points), "infill does not exceed perimeters ($pattern)";
+}
+
+{
+    my $config = Slic3r::Config->new_from_defaults;
+    $config->set('infill_only_where_needed', 1);
+    $config->set('bottom_solid_layers', 0);
+    $config->set('infill_extruder', 2);
+    $config->set('infill_extrusion_width', 0.5);
+    $config->set('fill_density', 40);
+    $config->set('cooling', 0);                 # for preventing speeds from being altered
+    $config->set('first_layer_speed', '100%');  # for preventing speeds from being altered
+    
+    my $test = sub {
+        my $print = Slic3r::Test::init_print('pyramid', config => $config);
+        
+        my $tool = undef;
+        my @infill_extrusions = ();  # array of polylines
+        Slic3r::GCode::Reader->new->parse(Slic3r::Test::gcode($print), sub {
+            my ($self, $cmd, $args, $info) = @_;
+            
+            if ($cmd =~ /^T(\d+)/) {
+                $tool = $1;
+            } elsif ($cmd eq 'G1' && $info->{extruding} && $info->{dist_XY} > 0) {
+                if ($tool == $config->infill_extruder-1) {
+                    push @infill_extrusions, Slic3r::Line->new_scale(
+                        [ $self->X, $self->Y ],
+                        [ $info->{new_X}, $info->{new_Y} ],
+                    );
+                }
+            }
+        });
+        return 0 if !@infill_extrusions;  # prevent calling convex_hull() with no points
+        
+        my $convex_hull = convex_hull([ map $_->pp, map @$_, @infill_extrusions ]);
+        return unscale unscale sum(map $_->area, @{offset([$convex_hull], scale(+$config->infill_extrusion_width/2))});
+    };
+    
+    my $tolerance = 5;  # mm^2
+    
+    $config->set('solid_infill_below_area', 0);
+    ok $test->() < $tolerance,
+        'no infill is generated when using infill_only_where_needed on a pyramid';
+    
+    $config->set('solid_infill_below_area', 70);
+    ok abs($test->() - $config->solid_infill_below_area) < $tolerance,
+        'infill is only generated under the forced solid shells';
+}
+
+{
+    my $config = Slic3r::Config->new_from_defaults;
+    $config->set('skirts', 0);
+    $config->set('perimeters', 1);
+    $config->set('fill_density', 0);
+    $config->set('top_solid_layers', 0);
+    $config->set('bottom_solid_layers', 0);
+    $config->set('solid_infill_below_area', 20000000);
+    $config->set('solid_infill_every_layers', 2);
+    $config->set('perimeter_speed', 99);
+    $config->set('external_perimeter_speed', 99);
+    $config->set('cooling', 0);
+    $config->set('first_layer_speed', '100%');
+    
+    my $print = Slic3r::Test::init_print('20mm_cube', config => $config);
+    my %layers_with_extrusion = ();
+    Slic3r::GCode::Reader->new->parse(Slic3r::Test::gcode($print), sub {
+        my ($self, $cmd, $args, $info) = @_;
+        
+        if ($cmd eq 'G1' && $info->{dist_XY} > 0 && $info->{extruding}) {
+            if (($args->{F} // $self->F) != $config->perimeter_speed*60) {
+                $layers_with_extrusion{$self->Z} = ($args->{F} // $self->F);
+            }
+        }
+    });
+    
+    ok !%layers_with_extrusion,
+        "solid_infill_below_area and solid_infill_every_layers are ignored when fill_density is 0";
+}
+
+{
+    my $config = Slic3r::Config->new_from_defaults;
+    $config->set('skirts', 0);
+    $config->set('perimeters', 3);
+    $config->set('fill_density', 0);
+    $config->set('layer_height', 0.2);
+    $config->set('first_layer_height', 0.2);
+    $config->set('nozzle_diameter', [0.35]);
+    $config->set('infill_extruder', 2);
+    $config->set('solid_infill_extruder', 2);
+    $config->set('infill_extrusion_width', 0.52);
+    $config->set('solid_infill_extrusion_width', 0.52);
+    $config->set('first_layer_extrusion_width', 0);
+    
+    my $print = Slic3r::Test::init_print('A', config => $config);
+    my %infill = ();  # Z => [ Line, Line ... ]
+    my $tool = undef;
+    Slic3r::GCode::Reader->new->parse(Slic3r::Test::gcode($print), sub {
+        my ($self, $cmd, $args, $info) = @_;
+        
+        if ($cmd =~ /^T(\d+)/) {
+            $tool = $1;
+        } elsif ($cmd eq 'G1' && $info->{extruding} && $info->{dist_XY} > 0) {
+            if ($tool == $config->infill_extruder-1) {
+                my $z = 1 * $self->Z;
+                $infill{$z} ||= [];
+                push @{$infill{$z}}, Slic3r::Line->new_scale(
+                    [ $self->X, $self->Y ],
+                    [ $info->{new_X}, $info->{new_Y} ],
+                );
+            }
+        }
+    });
+    my $grow_d = scale($config->infill_extrusion_width)/2;
+    my $layer0_infill = union([ map @{$_->grow($grow_d)}, @{ $infill{0.2} } ]);
+    my $layer1_infill = union([ map @{$_->grow($grow_d)}, @{ $infill{0.4} } ]);
+    my $diff = diff($layer0_infill, $layer1_infill);
+    $diff = offset2_ex($diff, -$grow_d, +$grow_d);
+    $diff = [ grep { $_->area > 2*(($grow_d*2)**2) } @$diff ];
+    is scalar(@$diff), 0, 'no missing parts in solid shell when fill_density is 0';
+}
+
+{
+    # GH: #2697
+    my $config = Slic3r::Config->new_from_defaults;
+    $config->set('perimeter_extrusion_width', 0.72);
+    $config->set('top_infill_extrusion_width', 0.1);
+    $config->set('infill_extruder', 2);         # in order to distinguish infill
+    $config->set('solid_infill_extruder', 2);   # in order to distinguish infill
+    
+    my $print = Slic3r::Test::init_print('20mm_cube', config => $config);
+    my %infill = ();  # Z => [ Line, Line ... ]
+    my %other  = ();  # Z => [ Line, Line ... ]
+    my $tool = undef;
+    Slic3r::GCode::Reader->new->parse(Slic3r::Test::gcode($print), sub {
+        my ($self, $cmd, $args, $info) = @_;
+        
+        if ($cmd =~ /^T(\d+)/) {
+            $tool = $1;
+        } elsif ($cmd eq 'G1' && $info->{extruding} && $info->{dist_XY} > 0) {
+            my $z = 1 * $self->Z;
+            my $line = Slic3r::Line->new_scale(
+                [ $self->X, $self->Y ],
+                [ $info->{new_X}, $info->{new_Y} ],
+            );
+            if ($tool == $config->infill_extruder-1) {
+                $infill{$z} //= [];
+                push @{$infill{$z}}, $line;
+            } else {
+                $other{$z} //= [];
+                push @{$other{$z}}, $line;
+            }
+        }
+    });
+    my $top_z = max(keys %infill);
+    my $top_infill_grow_d = scale($config->top_infill_extrusion_width)/2;
+    my $top_infill = union([ map @{$_->grow($top_infill_grow_d)}, @{ $infill{$top_z} } ]);
+    my $perimeters_grow_d = scale($config->perimeter_extrusion_width)/2;
+    my $perimeters = union([ map @{$_->grow($perimeters_grow_d)}, @{ $other{$top_z} } ]);
+    my $covered = union_ex([ @$top_infill, @$perimeters ]);
+    my @holes = map @{$_->holes}, @$covered;
+    ok sum(map unscale unscale $_->area*-1, @holes) < 1, 'no gaps between top solid infill and perimeters';
+}
+*/
+